For determining a mass flow of a medium flowing in a pipeline, especially a liquid or other fluid, often such measuring devices are used which, by means of a measuring transducer of vibration-type and a control and evaluation electronics connected thereto, effect in the fluid Coriolis forces and, derived from these forces, produce a measurement signal representing mass flow. Such measuring transducers, especially also their use in Coriolis mass flow meters, have been known already for a long time and are in industrial use. Thus e.g. EP-A 11 30 367, US-A 2005/0139015, U.S. Pat. Nos. 6,666,098, 6,477,902, 6,415,668, 5,549,009 or 5,287,754 describe Coriolis mass flow meters having, in each case, a measuring transducer of vibration-type, which measuring transducer reacts to a mass flow of a medium flowing in a pipeline and includes a transducer housing as well as an internal part arranged in the transducer housing. The internal part includes: At least one curved measuring tube vibrating during operation, at least at times, and serving for conveying the medium; as well as a counteroscillator affixed on the inlet end to the measuring tube for forming a first coupling zone and at the outlet end to the measuring tube for forming a second coupling zone. The counteroscillator essentially rests during operation, or it oscillates essentially equally-oppositely to the measuring tube, thus with equal frequency and opposite phase. The internal part is additionally held oscillatably in the transducer housing, at least by means of two connecting tube pieces, via which the measuring tube communicates with the pipeline during operation.
Curved, e.g. U, V or Ω shaped, vibrating measuring tubes can, as is known, when excited to bending oscillations according to a first eigenoscillation form, effect Coriolis forces in the medium flowing therethrough. Selected as a first eigenoscillation form of the measuring tube in the case of such measuring transducers is usually that eigenoscillation form wherein the measuring tube moves in the manner of a pendulum at a lowest natural resonance frequency about an imaginary longitudinal axis of the measuring transducer in the manner of a cantilever clamped at one end. The Coriolis forces produced in this way in the medium flowing therethrough lead, in turn, thereto, that the excited pendulum-like, cantilever oscillations of the so-called wanted mode are superimposed with bending oscillations according to at least one second eigenoscillation form of equal frequency. In the case of measuring transducers of the described kind, these cantilever oscillations compelled by Coriolis forces correspond to the so-called Coriolis mode, usually that eigenoscillation form at which the measuring tube also executes rotary oscillations about an imaginary vertical axis perpendicular to the longitudinal axis. Due to the superimposing of wanted and Coriolis modes, the oscillations of the measuring tube registered by means of the sensor arrangement at the inlet and outlet ends exhibit a measurable phase difference as a function also of the mass flow.
Frequently, the measuring tubes of such measuring transducers, e.g. installed in Coriolis mass flow meters, are excited during operation to an instantaneous resonance frequency of the first eigenoscillation form, especially at an oscillation amplitude regulated, or controlled, to a constant level. Since this resonance frequency, especially also, depends on the instantaneous density of the fluid, it is possible, e.g. in the case of usual commercially available Coriolis mass flow meters, also to measure, besides the mass flow, also the density of flowing fluids.
An advantage of a curved tube shape is that e.g. thermally related changes in length, especially also in the case of the use of measuring tubes having a high coefficient of thermal expansion, cause practically no, or only very small, mechanical stresses in the measuring tube itself and/or in the connected pipeline. A further advantage of curved measuring tubes is, however, to be seen in the fact that the measuring tube can be made relatively long and consequently a high sensitivity of the measuring tube can be achieved for the mass flow to be measured at a relatively short installed length and at relatively low exciter energy. These circumstances make it possible also to manufacture the measuring tube of materials of high coefficient of thermal expansion and/or high modulus of elasticity, i.e. materials such as e.g. stainless steel. In comparison thereto, in the case of measuring transducers of vibration-type having straight measuring tubes, the measuring tube is usually made of a material having at least a lower coefficient of thermal expansion and, as required, also a lower modulus of elasticity than stainless steel, in order to prevent axial stresses and achieve a sufficient sensitivity of measurement. Consequently, for this case, measuring tubes are preferably of titanium or zirconium, which are, however, on the basis of the higher cost of material and the usually also higher processing expense, much more expensive than those of stainless steel. Additionally, a measuring transducer with a single measuring tube has, as is known, compared to one with two measuring tubes flowed through in parallel, the additional great advantage that distributor pieces serving for the connecting of the measuring tubes with the pipeline are not necessary. Such distributor pieces are on the one hand complicated to manufacture and on the other hand also represent flow bodies having a marked inclination for the formation of accretions or for plugging.
Due to the mostly rather narrow band width of counteroscillators in the wanted mode, measuring transducers with a single curved measuring tube have, however, often for applications where density of the medium fluctuates over a wide range, especially also in comparison to such measuring transducers with two parallel measuring tubes, the disadvantage that, as a result of the imbalance of the internal part fluctuating with the density, the zero point of the measuring transducer and consequently also the measuring accuracy of the respective inline measuring device can equally fluctuate significantly and as, a result, can be correspondingly decreased. This is a result of, among other things, that also by means of the usually single counteroscillator, transverse forces can only be incompletely neutralized and, therefore, only incompletely kept away from the connected pipeline. Such transverse forces are induced in the measuring transducer due to alternating ended, lateral movements of the single measuring tube conveying the medium and are rather broadbanded as a result of strongly fluctuating medium densities, in comparison to the counterforces arising on the basis of the counteroscillator. The residual transverse forces can, in turn, lead to the fact that the above mentioned, internal part, moving, as a whole, in the manner of a pendulum about the longitudinal axis of the measuring transducer, begins also to oscillate laterally. These lateral oscillations of the internal part produce, correspondingly, also an additional elastic deformation of the connecting tube piece and can in this way effect also undesirable vibrations in the connected pipeline. Moreover, on the basis of such lateral oscillations of the internal part, it is also possible to provoke also cantilever oscillations in the measuring tube through which the fluid is not flowing. These are very similar to the Coriolis mode and, in any event, however, of equal frequency and consequently practically indistinguishable from the Coriolis mode, which, in turn, would make the measuring signal representing the actual mass flow unusable.
This arises also in the case of measuring transducers which are implemented according, for example, to the principle proposed in U.S. Pat. Nos. 5,705,754 or 5,287,754. In the case of measuring transducers described there, the transverse forces produced by, or on the part of, the vibrating, single measuring tube and which are rather mid or high frequency oscillatory forces, are attempted to be kept away from the pipeline by means of a single counteroscillator, which is rather heavy in comparison to the measuring tube and in any event is tuned to a higher frequency in comparison to the measuring tube, and, as required, by means of a relatively soft coupling of the measuring tube to the pipeline, thus, essentially by means of a mechanical low pass. Unfortunately, in this case, however, the mass of the counteroscillator required for achieving a sufficiently robust damping of the transverse forces rises more than proportionately with the nominal diameter of the measuring tube. This represents a great disadvantage for such measuring tubes of high nominal diameter, since a use of such components of high mass means, namely, always an increased cost of assembly, both in the manufacture, as well as also in the case of the installing of the measuring device into the pipeline. Moreover, in this case, it is only possible to assure, at great complexity, that the smallest eigenfrequency of the measuring transducer which, yes, also does become always lower with increasing mass, lies, after as before, very far from the likewise low eigenfrequencies of the connected pipeline. Consequently, a use of such a measuring transducer in industrially usable, inline measuring devices of the described kind, for example, Coriolis mass flow measuring devices, has long been rather limited to relatively low measuring-tube nominal diameters up to about 10 mm. Measuring transducers of the above described kind are moreover also sold on the part of the assignee itself under the mark “PROMASS”, series designation “A”, for a nominal diameter range of 1-4 mm and have proven themselves there, especially also in the case of applications with very low flow rates and/or high pressure.
In contrast, in the case of measuring transducers shown in U.S. Pat. Nos. 6,666,098, 6,477,902, or 5,549,009, the two, here essentially straight, connecting tube pieces are so oriented with respect to one another, as well as with respect to an imaginary longitudinal axis of the measuring tube, that the internal part, formed by means of the measuring tube and counteroscillator, as well as the oscillation exciters and oscillation sensors correspondingly applied thereto, can move, during operation, in a pendulum-like manner about the longitudinal axis. In other words, the entire internal part can execute pendulum oscillations during operation about the longitudinal axis L, conditioned on the, especially, density-dependent imbalances between measuring tube 10 and counteroscillator 20, which, depending on the way in which the imbalance shows itself, are of equal phase with the cantilever oscillations of the measuring tube 10 or of the counteroscillator 20. In such case, the torsional stiffnesses of the connecting tube pieces are preferably so tuned to one another and to the internal part carried by the two, that the internal part is suspended essentially rotationally softly about the longitudinal axis.
This is achieved, for example, in the case of U.S. Pat. No. 6,666,098, in such a manner that the torsional stiffness of the connecting tube pieces is so dimensioned that a respective eigenfrequency of a torsional oscillator inherently formed on the inlet end and on the outlet end by means of the respective connecting tube pieces and associated terminal mass fractions of the internal part which can be considered as essentially rigid and stable in form and oscillating about the longitudinal axis rotationally, lies in each case in the region of the oscillation frequency of the measuring tube oscillating in the wanted mode. Additionally, at least in the case of the measuring transducer proposed in the case of U.S. Pat. No. 6,666,098, measuring tube and counteroscillator are so tuned to one another that they oscillate at least in the wanted mode with approximately equal resonance frequency. Measuring transducers of the described kind are, furthermore, also sold by the assignee itself under the mark “PROMASS”, series designation “H”, for a nominal diameter range of 8-50 mm and have proven themselves there, especially also in the case of applications exhibiting a variable density of the medium to a considerable degree during operation. The pendulum-like movement of the internal part is, in this way, especially developed, or at least favored, such that both a measuring-tube center of mass spaced from the imaginary longitudinal axis, as well as also a center of mass of the counteroscillator spaced from the imaginary longitudinal axis, lie in a common region of the measuring transducer spanned by the imaginary longitudinal axis and the measuring tube.
However, investigations have in the meantime shown that the zero point of measuring transducers of the named kind can be subject, at very low mass flow rates and media deviating as to density considerably from the calibrated reference density, after, as before, to considerable fluctuations. Experimental investigations on measuring transducers configured according to U.S. Pat. No. 6,666,098, for which, as proposed, a relatively heavy counteroscillator has been used, have, it is true, led to the recognition that, in this way, there is quite a certain improvement of the null point stability and, as a result, an improvement of the measuring accuracy of inline measuring devices of the described kind, but, however, this has been achieved only to an unsatisfactory degree. In any case, in the configurations proposed in U.S. Pat. No. 6,666,098, a significant improvement of the measuring accuracy is essentially achievable only in the face of having to accept the already discussed disadvantages as discussed with reference to U.S. Pat. Nos. 5,705,754 or 5,287,754.